The present invention generally relates to apparatuses for reproducing information signals recorded on a recording medium, and more particularly to a reproducing apparatus which has a circuit for regulating the frequency characteristic so that number of errors in the reproduced data block is reduced, when reproducing the information signals from a recording medium on which the information signals are recorded as blocks of digital signals.
Recently, methods have been realized in which a train of digital signals is obtained by digitally modulating an information signal such as an audio signal, the train of digital signals is encoded by use of a predetermined modulating system, and the train of digital signals thus encoded is frequency modulated to record the information signal as a variation in geometrical configuration such as pits on a rotary recording medium (hereinafter simply referred to as a disc), and then to reproduce the above information signal.
However, there are cases where errors are introduced in the reproduced signal, upon reproduction of the information signal from the above disc in which the information signal is recorded. When these errors exist, noise is introduced in the reproduced sound which is obtained by converting the above reproduced signal into an analog signal. Accordingly, the reproduced sound thus obtained in this case is of a low quality, and unpleasant to the ear.
The errors introduced in the above reproduced signal are mainly due to causes such as deformation in the configuration and depth of pits of the disc introduced in a manufacturing process of the disc, and undesirable deviation from the standard value in the electrode configuration of a reproducing stylus of a reproducing apparatus and in the mounting direction of the reproducing stylus. Moreover, degradation is introduced in the reproduced frequency characteristic at the inner peripheral part of the disc, since the relative scanning linear velocity between the reproducing stylus and the disc decreases as the reproducing stylus moves towards the inner peripheral part of the disc from the outer peripheral part of the disc. Therefore, the above described errors are also introduced due to the degradation in the above frequency characteristic.
To describe the above in a more detailed manner, in general, in an audio disc in which the bit rate of the train of digital signals is relatively high, namely six Mbit/second, for example, the band of the frequency spectrum of the recorded signal is wide. Accordingly, when a signal having such a wide frequency spectrum is frequency modulated, a high signal-to-noise (S/N) ratio cannot be obtained, since the modulation index decreases especially in the high frequency range components.
On the other hand, in the above described type of a disc, the signal reproduced from the disc is generally reproduced by always maintaining the rotational speed of the disc at a constant value, regardless of the reproducing position of the pickup stylus. Hence, the relative linear velocity between the reproducing stylus and the disc at the inner peripheral part of the disc is lower than that at the outer peripheral part of the disc. Accordingly, the level of the high frequency range components decreases as the reproducing stylus reproduces the inner peripheral part of the disc, since the modulation index in the high frequency range components decreases as described above, and since the relative linear velocity between the reproducing stylus and the disc is low at the inner peripheral part of the disc. Especially in the inner peripheral part of the disc, the carrier-to-noise (C/N) ratio of the frequency-modulated signal decreases. Furthermore, the level of the reproduced signal decreases especially in the high frequency range components with respect to the level of the original signal.
Accordingly, in the conventional reproducing apparatus, distortion is introduced in the output of the demodulator, due to the decrease in the level of the high frequency range components. As a result, it becomes difficult to discriminate the data, and code errors are accordingly introduced. Therefore, in the conventional apparatus, there was a disadvantage in that the information signal could not be reproduced accurately throughout the entire disc.
As described above, the relative linear velocity between the reproducing stylus and the disc at the inner peripheral part of the disc is lower than that at the outer peripheral part of the disc. Hence, in order to reproduce the information signal under the same condition throughout the entire disc, the length of the pits along the rotational direction of the disc must be formed so that the pits are shorter at the inner peripheral part of the disc compared to the pits formed at the outer peripheral part of the disc.
Upon manufacturing of the disc, the pits are unlikely to be formed sharply at the peripheral edge portions of the pits, due to limitations in the accuracy of the beam upon irradiation to form these pits, and the like. Thus, it is difficult to form rows of pits having a short length, with high accuracy, at the inner peripheral part of the disc. Hence, when the signal waveform having a sinusoidal wave shape obtained by wave-shaping the signal reproduced by the reproducing stylus in an equalizer and the like is examined, the reproduced waveform obtained from the pit at the outer peripheral part of the disc is normal while the reproduced waveform obtained from the pit at the inner peripheral part of the disc is distorted, for exactly the same signal, for example. This distortion in the reproduced waveform is introduced since the reproduced signal level obtained from a pit at the inner peripheral part of the disc decreases especially in the high frequency range, in addition to the above described causes.
Therefore, in the conventional reproducing apparatus, carrier leak is introduced in the output of the demodulator due to the above distortion in the waveform. As a result, it becomes difficult to discriminate the data, and code errors are introduced. Hence, there was a disadvantage in that the information signal could not be accurately reproduced throughout the entire disc.
The above errors introduced in the reproduced signal can be reduced by suitable adjusting or regulating the frequency characteristic of the reproducing apparatus. Accordingly, the present invention was matured by taking this point into consideration.